Ozone is a powerful disinfectant and is used to oxidize biodegradable organic contaminants from drinking water. It is useful in removing the taste and odor-causing compounds that are produced by blue-green algae in the surface water. Ozone is also used for tertiary treatment to remove the trace contaminants from filtered municipal waste water before reuse as indirect potable water or being discharged to environmentally sensitive regions. For the synthetic organic contaminants such as MTBE, TCE, 1,4 dioxane etc. typically found in chemical contaminated ground water sites, an advanced oxidation process is used.
Ozone can be used in combination with hydrogen peroxide and/or catalysts to produce hydroxyl radicals which oxidize the recalcitrant organic contaminants. Hydroxyl radicals are produced by the reaction between ozone and hydrogen peroxide or a catalyst in the aqueous phase. This type of treatment is referred to in the industry as an “advanced oxidation” process.
Ozone gas is commonly produced in a corona discharge-based generator from air or high purity oxygen. The typical concentration of ozone in gas phase ranges from 3 to 14%, depending on the generator power and concentration of oxygen in the gas feed used for ozone generation. Ozone-based water treatment processes depend upon transfer of ozone from the gas phase to the water phase for oxidation of organic contaminants. Various processes have been used to transfer ozone from gas phase to liquid phase for the purposes of water treatment.
One such known process is a bubble column or basin reactor, which comprises a large column or basin and gas diffusers located at the bottom of the column or basin. The column or basin is filled with water and ozone gas is introduced through the gas diffusers. Fine bubbles of ozone gas rise through the water in the column or basin, which promotes dissolution of the ozone into the water (also referred to herein as “ozone transfer”). Ozone transfer efficiency can be improved by capturing and recirculating undissolved ozone from the top of the column or basin and/or passing the ozone through a series of columns or basins using baffles. One problem with this dissolution method is that the diffusion pores of the gas diffuser typically clog over time, which adversely impacts performance. Another problem with a diffuser-based ozone transfer process is that large and deep basins are required for effective transfer of ozone to water. In addition, diffuser-based ozone transfer processes are relatively inefficient methods of ozone transfer.
Another known ozone transfer method is the use of a venturi ejector, in which water flows through the venturi and ozone gas is injected at the throat of the venturi. This venturi-based method can only be used in systems with relatively low water flow rates. In systems that operate at relatively large flow rates, a portion of the water can be diverted into a “slip stream” on which the venturi is located. The slip stream is then injected back into the main stream and mixed into the main stream by turbulent flow. The diverted stream venturi method is typically only effective for relatively low-dose ozone transfer (e.g., 10 mg/L or less).
In another variation of venturi-based ozone transfer, static mixers can be used downstream from the injector to achieve additional mixing of ozone in the water phase. The system is simpler to design as it has no moving parts. But the mixing and gas dispersion for good ozone transfer through a static mixer requires a highly turbulent flow of gas and liquid. This leads to a higher pressure drop and can only be operated in a narrow range of water and gas flow rates.
There have been attempts to perform ozone transfer using turbine contactors, which operate by aspirating gas through hollow turbine shafts and agitators. Turbine contactors do not appear to be well-suited to ozone transfer applications for several reasons. As compared to the ozone transfer methods described above, turbine contactors have relatively high power requirements. In addition, the ratio of ozone gas to water entering the turbine contactor must be kept relatively constant for efficient operation, which limits the ability to adjust ozone dosing. Turbine contactors are not well-suited for catalytic ozonation because the powdered catalyst will plug the channels through which the ozone gas is aspirated.
Packed columns are rarely used for ozone transfer because this type of reactor has very low ozone transfer efficiency, and therefore, a very tall column is required to achieve typical ozone dosing. Packed columns also have low void volume, which limits the water flow rate through a given diameter column. Packed columns can be used for fixed bed catalytic reactions with ozone but, due to low mass transfer efficiency of ozone, are expensive to build and operate.
Impinging jets have been used to enhance mixing between gas and liquid phases in ozone transfer systems. In such systems, a high-velocity jet of two phase flow is impacted with another jet or with a stationary surface. A portion of the water may be recycled through the jets. In addition, undissolved ozone may be captured downstream in a phase separator and recycled through the jets. Impinging jets can be used as the sole mixing reactor, or can be used in combination with other mixing reactors. The design and operation of an ozone transfer system including impinging jets is complex due to the need for precision location of the impact zones. In addition, the jets have relatively high power requirements and the rate of flow rates that can be accommodated by this type of system is limited.
Accordingly, there is a need for an improved method of ozone transfer that overcomes the deficiencies of the methods of the prior art.